Support index-only scans on our hnsw access method

[Ngalstyan4]

Note: this is quite an involved issue. It will require understanding of the index storage format and structures on postgres blocks.

Relevant links:

• Postgres CREATE INDEX docs(search for INCLUDE)
• Index-only scans and covering indexes
• Index-only scans and postgres visibility map

[Posrabi]

So if I understand this correctly, the hnsw index type does not support index-only and we want to implement it? Ref:

The index type must support index-only scans. B-tree indexes always do. GiST and SP-GiST indexes support index-only scans for some operator classes but not others. Other index types have no support. The underlying requirement is that the index must physically store, or else be able to reconstruct, the original data value for each index entry. As a counterexample, GIN indexes cannot support index-only scans because each index entry typically holds only part of the original data value.

[Posrabi]

And this can be achieved by properly supporting the INCLUDE expression on the CREATE INDEX statement.

[Posrabi]

Access methods API descriptions: https://www.postgresql.org/docs/9.5/catalog-pg-am.html
I think as a first step we can implement amcanreturn and then support INCLUDE later. Effectively if we only query the column cover by the index then we would still be doing an index-only scan.